Generally, the power delivered by an internal-combustion engine is a function of the amount of air fed into the combustion chamber of this engine, an amount of air which is itself proportional to the density of this air. As it is well known, if high power is required, the intake air can be compressed, by any compression means such as a turbosupercharger, before it is fed into the cylinder of the engine, to obtain air supercharging in this cylinder.
In order to increase this amount of air in the cylinder and, in case of a spark-ignition engine, to reduce the engine knock sensitivity, the residual burnt gases in the combustion chamber are discharged, before the end of the engine exhaust phase, to be replaced by supercharged air, a stage that is commonly referred to as burnt gas scavenging.
This scavenging can be obtained by carrying out, in the vicinity of the top dead center of a cylinder ending its exhaust phase and starting its intake phase, overlapping of the exhaust and intake valves, as described in U.S. Pat. No. 4,217,866 for an indirect-injection engine. This overlap is obtained by opening simultaneously, for some degrees to some ten degrees of crankshaft rotation angle, the exhaust and intake valves of the cylinder at the end of the exhaust phase and in the vicinity of the top dead center (TDC) of its piston. This cylinder therefore comprises a specific intake means for non-carbureted supercharged air as a complement to a conventional intake means for feeding carbureted supercharged air into the combustion chamber, each means consisting of a pipe and of a valve. In this configuration, this burnt gas scavenging is carried out by overlap of the exhaust valves and of the non-carbureted supercharged air intake valve, so that this air, whose pressure is higher than at the exhaust, is fed into the combustion chamber and scavenges these gases to discharge them through the exhaust valves and thus occupies the volume freed by these gases. At the end of the scavenging stage, the exhaust valves close and carbureted supercharged air is fed into the combustion chamber through the other intake means provided with a fuel nozzle.
Although this type of engine runs satisfactorily, it involves drawbacks that are by no means insignificant.
In fact, to carry out such scavenging, the supercharged air intake pressure has to be higher than the burnt gas exhaust pressure in the vicinity of the top dead center of the piston so as to drive these exhaust gases towards the exhaust valve and to replace them by supercharged air fed through the intake valve.
However, the use of a conventional turbosupercharger, which comprises a single inlet for the exhaust gas in the turbine, does not allow, in some cases, to carry out this scavenging for engines having at least two cylinders and more particularly for engines with four cylinders.
In fact, the exhaust gases leaving each cylinder through the exhaust means are sent to the single inlet of the turbine of the supercharger through a line connecting, directly or indirectly by means of an exhaust manifold, all the exhausts of all the cylinders to the turbine inlet.
Thus, at the start of each exhaust phase of a cylinder and upon opening of the exhaust valves, the exhaust pressure at the turbine inlet undergoes an increase for some crankshaft rotation angle degrees. This pressure increase has repercussions, through the manifold, on the exhaust valves of the other cylinders.
In a four-stroke engine having at least two cylinders, as in a four-cylinder engine, the working cycle of this engine is such that the start of the intake phase of a cylinder with opening of the intake valve starts substantially at the same time as the start of the exhaust phase of another cylinder with opening of the exhaust valve. Therefore, when the cylinder is in the intake phase with burnt gas scavenging by intake and exhaust valves overlap, the exhaust gases from the cylinder starting its exhaust phase communicate through the exhaust manifold with the cylinder starting its intake phase for which the exhaust valve is open to provide this scavenging. The exhaust gases present in the manifold are therefore at a higher pressure than the intake pressure and prevent, during valve overlap, discharge of the burnt gases through the exhaust valve of the cylinder starting its intake phase.
To overcome this problem, it is well-known to use a specific turbosupercharger referred to as double-flow supercharger. In this type of supercharger, the exhaust gas inlet at the level of the turbine is divided in two sections, a first section connected, directly or by means of a manifold, to the exhausts of part of the cylinders and a second section connected to the exhausts of the other cylinders. In order to avoid the aforementioned drawback, each section is connected to cylinders for which an intake phase of a cylinder and an exhaust phase of another cylinder do not occur simultaneously.
Thus, when one of the cylinders connected to a section of the supercharger starts its intake phase with burnt gas scavenging, the pressure of the exhaust gases of the cylinder starting its exhaust phase, which is connected to the other section of the supercharger, cannot hinder the burnt gas scavenging process.
This double-flow supercharger technology, although it gives satisfaction, is of a much higher cost than conventional single-inlet superchargers.
The present invention aims to overcome the aforementioned drawbacks by means of an engine control allowing to carry out any time scavenging of the burnt gases of a cylinder and to use an air supercharged by means of a single-inlet supercharger.